Every Parampara has it own Guru mantra and any mantra communicated secretly and verbally by a Guru becomes a Guru-mantra for the person to whom it is communicated. In vedic system and in our culture the place of Guru is even higher than God. It is symbolised as:

A person who worship his Guru, immediately on getting up and before steeping out of bed daily followed by worship of five deities Ganesh, Surya Vishnu Shakti and Shiva (preferably in that order) shall have a smooth and happy life.

Guru Beej Akshara - (OM)

This is naisargika Tarak Mantra and is also called Tara Beej. It is prescribed for all worshippers of Guru and in Guru Granth Sahib (Ek-Omkar) & in Dakshinamurti strota etc.